Unable to use Intellij with a generated sources folder

You can just change the project structure to add that folder as a "source" directory.

Project Structure → Modules → Click the generated-sources folder and make it a sources folder.

Or:

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>build-helper-maven-plugin</artifactId>
    <version>1.4</version>
    <executions>
        <execution>
            <id>test</id>
            <phase>generate-sources</phase>
            <goals>
                <goal>add-source</goal>
            </goals>
            <configuration>
                <sources>
                    <source>${basedir}/target/generated-sources</source>
                </sources>
            </configuration>
        </execution>
    </executions>
</plugin>

I'm using Maven (SpringBoot application) solution is:

1. Right click project folder
2. Select Maven
3. Select Generate Sources And Update Folders

Then, Intellij automatically import generated sources to project.